colon - Vector creation, array subscripting, and for-loop

        iteration - MATLAB ([original](https://in.mathworks.com/help/matlab/ref/double.colon.html)) ([raw](?raw))

Vector creation, array subscripting, and for-loop iteration

Syntax

x = j:k x = j:i:k A(:,n) A(m,:) A(:) A(j:k)

Description

The colon is one of the most useful operators in MATLAB®. It can create vectors, subscript arrays, and specifyfor iterations.

[x](#bvg1wnp-x) = [j](#bvg1wnp-j):[k](#bvg1wnp-k) creates a unit-spaced vector x with elements[j,j+1,j+2,...,j+m] where m = fix(k-j). If j and k are both integers, then this is simply [j,j+1,...,k].

example

[x](#bvg1wnp-x) = [j](#bvg1wnp-j):[i](#bvg1wnp-i):[k](#bvg1wnp-k) creates a regularly-spaced vector x usingi as the increment between elements. The vector elements are roughly equal to [j,j+i,j+2*i,...,j+m*i] where m = fix((k-j)/i). However, if i is not an integer, then floating point arithmetic plays a role in determining whethercolon includes the endpoint k in the vector, since k might not be exactly equal to j+m*i.

example

x = colon(j,k) and x = colon(j,i,k) are alternate ways to execute the commands j:k andj:i:k, but are rarely used. These syntaxes enable operator overloading for classes.

A(:,n), A(m,:),A(:), and A(j:k) are common indexing expressions for a matrix A that contain a colon. When you use a colon as a subscript in an indexing expression, such asA(:,n), it acts as shorthand to include_all_ subscripts in a particular array dimension. It is also common to create a vector with a colon for the purposes of indexing, such as A(j:k). Some indexing expressions combine both uses of the colon, as in A(:,j:k).

Common indexing expressions that contain a colon are:

• A(:,n) is the nth column of matrix A.
• A(m,:) is the mth row of matrixA.
• A(:,:,p) is the pth page of three-dimensional array A.
• A(:) reshapes all elements of A into a single column vector. This has no effect if A is already a column vector.
• A(:,:) reshapes all elements ofA into a two-dimensional matrix. This has no effect if A is already a matrix or vector.
• A(j:k) uses the vector j:k to index into A. If A is a vector, thenA(j:k) has the same orientation asA. If A is a matrix, thenA(j:k) is a row vector.
• A(:,j:k) includes all subscripts in the first dimension but uses the vector j:k to index in the second dimension. This returns a matrix with columns [A(:,j), A(:,j+1), ..., A(:,k)].

example

Examples

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Create a unit-spaced vector of numbers between 1 and 10. The colon operator uses a default increment of +1.

x = 1×10

 1     2     3     4     5     6     7     8     9    10

Create vectors that increment or decrement by a specified value.

Create a vector whose elements increment by 0.1.

x = 1×11

     0    0.1000    0.2000    0.3000    0.4000    0.5000    0.6000    0.7000    0.8000    0.9000    1.0000

Create a vector whose elements decrement by -2.

Examine several ways to index a matrix using a colon :.

Create a 3-by-3 matrix. Index the first row.

A = 3×3

 8     1     6
 3     5     7
 4     9     2

Index the second and third column.

Reshape the matrix into a column vector.

ans = 9×1

 8
 3
 4
 1
 5
 9
 6
 7
 2

In the context of a for-loop, the colon specifies the loop iterations.

Write a for-loop that squares a number for values of n between 1 and 4.

Input Arguments

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Starting vector value, specified as a real numeric scalar. If j < k so that the output vector is not empty, thenj is the first element in the vector.

Example: x = 0:5

Example: x = 0:0.5:5

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | char | datetime | duration

Ending vector value, specified as a real numeric scalar.k is the last value in the vector only when the increment lines up to exactly land on k. For example, the vector 0:5 includes 5 as the last value, but0:0.3:1 does not include the value 1 as the last value since the increment does not line up with the endpoint.

Example: x = 0:5

Example: x = 0:0.5:5

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | char | datetime | duration

Increment between vector elements, specified as a real numeric scalar.

Example: x = 0:0.5:5

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | char | datetime | duration

Output Arguments

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Regularly-spaced vector, returned as a row vector. If j > k, then x = j:k is an empty matrix. More generally, the syntax x = j:i:k returns an empty matrix when:

• i, j, ork is an empty input
• i == 0
• i > 0 and j > k
• i < 0 and j < k

Tips

• The for reference page has a description of how to use : in the context of loop statements.
• linspace is similar to the colon operator :, but it gives direct control over the number of points and always includes the endpoints. The sibling function logspace generates logarithmically spaced values.
• When you create a vector to index into a cell array or structure array (such as _`cellName`_{:} or_`structName`_(:)._`fieldName`_), MATLAB returns multiple outputs in a comma-separated list. For more information, see How to Use Comma-Separated Lists.

Extended Capabilities

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Usage notes and limitations:

• Complex inputs are not supported.
• The input i cannot have a logical value.
• Vector inputs are not supported.
• Inputs must be constants.
• Uses single-precision arithmetic to produce single-precision results.

The colon function supports GPU array input with these usage notes and limitations:

• To run this function on a GPU and obtain a gpuArray output, use any of the following syntaxes:
  x = gpuArray.colon(j,k)
  x = gpuArray.colon(j,i,k)
  Alternatively, you can also pass gpuArray objects directly to the colon operator:
  x = gpuArray(j):gpuArray(k)
  x = gpuArray(j):gpuArray(i):gpuArray(k)
• 64-bit integers are not supported.

For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Version History

Introduced before R2006a

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colon now returns an error when creating vectors if one or more operands are not scalar. For example, expressions like [1 2 3]:2:10 now error. Previously, colon used the first element of any nonscalar operands to evaluate the expression.

To preserve the behavior of previous releases, update your code to specify the first element of any nonscalar operands in place of those nonscalar operands. For example, this code now returns an error because size(A) is nonscalar.

A = randn(3,4); v = 1:size(A)

In previous releases, this code was equivalent tov = 1:3 because size(A) returned the vector [3 4], and colon used only the size of the first dimension of A to evaluate the expression. To achieve the same result, explicitly specify the size of the first dimension ofA as the operand.

As another example, this code also returns an error because id is nonscalar.

id = find([1 2 3 1 2 3] == 3); v = 1:id

In previous releases, this code was equivalent tov = 1:3 because find returned the vector[3 6] and colon used only the first element of this returned vector to evaluate the expression. To achieve the same result, explicitly specify the first element of the vector as the operand.

Alternatively, you can directly specify that find return only the first index.

id = find([1 2 3 1 2 3] == 3,1,"first"); v = 1:id

If you have a vector of sorted values, you can also use the min function to access the first element of the vector. For example, this code now returns an error because strfind returns a vector.

path = 'matlab/users/username/examples'; topDirname = path(1:strfind(path,'/')-1)

To achieve the same result as in previous releases, use the min function on the sorted vector returned bystrfind.

topDirname = path(1:min(strfind(path,'/'))-1)

colon now issues a warning when creating vectors if one or more operands are not scalar. For example, expressions like [1 2 3]:2:10 now warn. This warning will become an error in a future release, removing support for nonscalar operands. Previously,colon used the first element of any nonscalar operands to evaluate the expression.